Body wash products can be very inefficient delivery vehicles for depositing hydrophobic actives and moisturizers on the skin's surface, since most of the hydrophobic actives are rinsed away during the washing process. The majority of body wash products consist of entangled rod-like micelle formulas. Current oil-in-water body wash formulas do not form stable products at high oil concentrations, since the oils and product aqueous base have significantly different densities. Since micellar based body washes can have high viscosities, but not long range structure (substantially larger than the dimensions of the surfactant micelles themselves), products containing high concentrations of oil will be unstable and phase separate with time. One solution to this problem is to use oil-in-water emulsion systems to incorporate oil into surfactant systems. However, these systems can require heat to make, may be unstable at higher oil concentrations, and may result in a significant impairment of cleansing properties at low surfactant concentrations.
Spherulite based formulas with lamellar surfactant phases which form structured systems have been described. Examples include U.S. Pat. Nos. 5,661,189; 5,965,500; and 6,174,846. These spherulite systems are used primarily to enhance the stability of body wash formulas containing significant amounts of emollients and oils through a significant reduction in the rate of diffusion of oil droplets in the surfactant media. Thus, increased concentrations of oils can be incorporated into the products with the potential to delivery higher concentrations to the skin's surface, when compared to rod like micelle formulations.
The rheological behavior of all surfactant solutions, including liquid cleansing solutions, is strongly dependent on the microstructure, i.e., the shape and concentration of micelles or other self-assembled structures in solution. When there is sufficient surfactant to form micelles (concentrations above the critical micelle concentration or CMC), for example, spherical, cylindrical (rod-like) or discoidal micelles may form. As surfactant concentration increases, ordered liquid crystalline phases such as lamellar phase, hexagonal phase or cubic phase may form. The lamellar phase, for example, consists of alternating surfactant bilayers and water layers. These layers are not generally flat but fold to form spherical onion like structures called vesicles or liposomes. The hexagonal phase, on the other hand, consists of long cylindrical micelles arranged in a hexagonal lattice. In general, the microstructure of most personal care products consist of either spherical micelles; rod micelles; or a lamellar dispersion.
As noted above, micelles may be spherical or rod-like. Formulations having spherical micelles tend to have a low viscosity and exhibit newtonian shear behavior (i.e., viscosity stays constant as a function of shear rate; thus, if easy pouring of product is desired, the solution is less viscous and, as a consequence, it doesn't suspend as well). In these systems, the viscosity increases linearly with surfactant concentration.
Rod micellar solutions are more viscous because movement of the longer micelles is restricted. At a critical shear rate, the micelles align and the solution becomes shear thinning. Addition of salts increases the size of the rod micelles thereof increasing zero shear viscosity (i.e., viscosity when sitting in bottle) which helps suspend particles but also increases critical shear rate (point at which product becomes shear thinning; higher critical shear rates means product is more difficult to pour).
Lamellar dispersions differ from both spherical and rod-like micelles because they can have high zero shear viscosity (because of the close packed arrangement of constituent lamellar droplets), yet these solutions are very shear thinning (readily dispense on pouring). That is, the solutions can become thinner than rod micellar solutions at moderate shear rates.
In formulating liquid cleansing compositions, therefore, there is the choice of using rod-micellar solutions (whose zero shear viscosity, e.g., suspending ability, is not very good and/or are not very shear thinning); or lamellar dispersions (with higher zero shear viscosity, e.g. better suspending, and yet are very shear thinning).
To form such lamellar compositions, however, some compromises have to be made. First, generally higher amounts of surfactant are required to form the lamellar phase. Thus, it is often needed to add auxiliary surfactants and/or salts which are neither desirable nor needed. Second, only certain surfactants will form this phase and, therefore, the choice of surfactants is restricted.
In short, lamellar compositions are generally more desirable (especially for suspending emollient and for providing consumer aesthetics), but more expensive in that they generally require more surfactant and are more restricted in the range of surfactants that can be used.
When rod-micellar solutions are used, they also often require the use of external structurants to enhance viscosity and to suspend particles (again, because they have lower zero shear viscosity than lamellar phase solutions). For this, carbomers and clays are often used. At higher shear rates (as in product dispensing, application of product to body, or rubbing with hands), since the rod-micellar solutions are less shear thinning, the viscosity of the solution stays high and the product can be stringy and thick. Lamellar dispersion based products, having higher zero shear viscosity, can more readily suspend emollients and are typically creamier. Again, however, they are generally more expensive to make (e.g., they are restricted as to which surfactants can be used and often require greater concentration of surfactants).
In general, lamellar phase compositions are easy to identify by their characteristic focal conic shape and oily streak texture while hexagonal phase exhibits angular fan-like texture. In contrast, micellar phases are optically isotropic.
It should be understood that lamellar phases may be formed in a wide variety of surfactant systems using a wide variety of lamellar phase “inducers” as described, for example, in PCT publication, WO 97/05857. Generally, the transition from micelle to lamellar phase is a function of effective average area of headgroup of the surfactant, the length of the extended tail, and the volume of tail. Using branched surfactants or surfactants with smaller headgroups or bulky tails are all effective ways of inducing transitions from rod micellar to lamellar.
U.S. Pat. No. 5,661,189 directed to a detergent composition, teaches an aqueous liquid cleansing and moisturizing composition comprising a surface active agent selected from anionic, nonionic, zwitterionic and cationic surface active agents and mixtures thereof; an benefit agent having a weight average particle size in the range 50 to 500 microns; and a thickening agent. The thickening agent is added to the benefit agent in amount from 1 to 50% wt, based on the benefit agent.
U.S. Pat. No. 5,965,500 for a stable liquid composition comprising high levels of emollients, teaches the use of high foaming aqueous liquid compositions with levels of oil/emollient equal to or in excess of level of surfactant. Good levels of foam can be maintained at such high levels of emollient. In addition to surfactant and emollient, compositions also preferably comprise C12-24 fatty acid and/or cationic polymer.
U.S. Pat. No. 6,174,846 for a liquid composition with enhanced low temperature stability, teaches the use of liquid cleansing compositions in a lamellar phase. Use of minimum amounts of defined polymeric hydrophilic emulsifier in combination with a lamellar phase inducing structurant has been found to enhance both initial viscosity and free thaw (low temperature) viscosity/stability.
U.S. Patent Application Publication Number 2003/018046 for a stable surfactant composition for suspending components teaches free-flowing surfactant composition comprising at least one anionic surface-active agent, an alkanolamide, an electrolyte, and water is described. In particular, the composition is a surfactant composition that has free-flowing non-Newtonian shear thinning properties and the ability to suspend components and is stable under at least one freeze/thaw cycle.
It is to be noted, however, that current spherulite products require improvement in delivering oil to substrates such as skin, hair or wool. Thus, there remains a need to provide enhanced delivery of an oil phase to a substrate such as skin, hair or wool by incorporating the oil into the spherulite formula at a later stage in the manufacturing process, and protecting the oil from excessive emulsification.
There is also a need to enhance the delivery of the oil phase to the skin by incorporating the oil into the spherulite formula at a later stage in the manufacturing process, in conjunction with increased salt concentrations.